In a combustion environment like in a boiler where crude oil, heavy oil, and the like are used as fuel or in refuse incineration, V.sub.2 O.sub.5, Na.sub.2 SO.sub.4, NaCl, KCl and CaSO.sub.4 present in the fuel or formed during combustion adhere to corrosion products such as oxidized scales, and deposit thereon to form a molten salt environment. As a result, a type of corrosion termed vanadium attack and hot corrosion takes place.
Literature on related techniques (e.g., Corrosion, 42, 568 (1986); Iron and Steel, 67, 996 (1981)) illustrates that alloy components such as Cr and Ni have a corrosion resistance to a certain degree to such types of corrosion, particularly to vanadium attack. Moreover, Unexamined Japanese Patent (Kokai) No. 58-177438 discloses an austenitic stainless steel having an improved high temperature corrosion resistance.
On the other hand, many processes are known for producing a double layer tube. For example, there is a process for producing a welded multilayer steel tube, wherein a cladding alloy is temporarily bonded to a carbon steel or low alloy steel, the bonded material is further hot rolled to give a so-called clad steel sheet or plate, and the sheet or plate is subjected to submerged arc welding. There is also a conventional process for producing a multilayer steel tube, wherein a final product is directly clad with a metal. For example, Unexamined Japanese Patent Publication (Kokai) No. 61-223106 discloses a process for directly producing a final product, a multilayer steel tube, comprising cladding a metal material with high alloy powder by hot isostatic pressing. Furthermore, Unexamined Japanese Patent Publication (Kokai) No. 64-17806 discloses a process for producing a multilayer steel tube, though the method for producing a multilayer billet is not clearly described.